By virtue of excellent magnetic properties, sintered Nd base magnets find an ever increasing range of application. In the field of rotary machines such as motors and power generators, permanent magnet rotary machines using sintered Nd base magnets have recently been developed in response to the demands for weight and profile reduction, performance improvement, and energy saving.
The permanent magnets within the rotary machine are exposed to elevated temperature due to the heat generation of windings and iron cores and kept susceptible to demagnetization by a magnetic field of opposite direction from the windings. There thus exists a need for a sintered Nd base magnet having heat resistance, a certain level of coercive force serving as an index of demagnetization resistance, and a maximum remanence serving as an index of magnitude of magnetic force.
Several methods are known to improve coercive force.
An increase in the remanence of sintered Nd base magnets can be achieved by increasing the volume factor of Nd2Fe14B compound and improving the crystal orientation. To this end, a number of modifications have been made on the process. For increasing coercive force, there are known different approaches including grain refinement, the use of alloy compositions with greater Nd contents, and the addition of effective elements. The currently most common approach is to use alloy compositions in which Dy or Tb substitutes for part of Nd. Substituting these elements for Nd in the Nd2Fe14B compound increases both the anisotropic magnetic field and the coercive force of the compound. The substitution with Dy or Tb, on the other hand, reduces the saturation magnetic polarization of the compound. Therefore, as long as the above approach is taken to increase coercive force, a loss of remanence is unavoidable.
In sintered Nd base magnets, the coercive force is given by the magnitude of an external magnetic field created by nuclei of reverse magnetic domains at grain boundaries. Formation of nuclei of reverse magnetic domains is largely dictated by the structure of the grain boundary in such a manner that any disorder of grain structure in proximity to the boundary invites a disturbance of magnetic structure, helping formation of reverse magnetic domains. It is generally believed that a magnetic structure extending from the grain boundary to a depth of about 5 nm contributes to an increase of coercive force. See Non-Patent Document 1: K. D. Durst and H. Kronmuller, “THE COERCIVE FIELD OF SINTERED AND MELT-SPUN NdFeB MAGNETS,” Journal of Magnetism and Magnetic Materials, 68 (1987), 63-75.
The inventors discovered that when a slight amount of Dy or Tb is concentrated only in proximity to the interface of grains for thereby increasing the anisotropic magnetic field only in proximity to the interface, the coercive force can be increased while suppressing a decline of remanence (Patent Document 1: JP-B H05-31807). Further the inventors established a method of producing a magnet comprising separately preparing a Nd2Fe14B compound composition alloy and a Dy or Tb-rich alloy, mixing and sintering (Patent Document 2: JP-A H05-21218). In this method, the Dy or Tb-rich alloy becomes a liquid phase during the sintering step and is distributed so as to surround the Nd2Fe14B compound. As a result, substitution of Dy or Tb for Nd occurs only in proximity to grain boundaries of the compound, which is effective in increasing coercive force while suppressing a decline of remanence.
The above method, however, suffers from some problems. Since a mixture of two alloy fine powders is sintered at a temperature as high as 1,000 to 1,100° C., Dy or Tb tends to diffuse not only at the interface of Nd2Fe14B crystal grains, but also into the interior thereof. An observation of the structure of an actually produced magnet reveals that Dy or Tb has diffused in a grain boundary surface layer to a depth of about 1 to 2 microns from the interface, and the diffused region accounts for a volume fraction of 60% or above. As the diffusion distance into crystal grains becomes longer, the concentration of Dy or Tb in proximity to the interface becomes lower. Lowering the sintering temperature is effective to minimize the excessive diffusion into crystal grains, but not practically acceptable because low temperatures retard densification by sintering. An alternative approach of sintering a compact at low temperature under a pressure applied by a hot press or the like is successful in densification, but entails an extreme drop of productivity.
Another method for increasing coercive force is known in the art which method comprises machining a sintered magnet into a small size, applying Dy or Tb to the magnet surface by sputtering, and heat treating the magnet at a lower temperature than the sintering temperature for causing Dy or Tb to diffuse only at grain boundaries. See Non-Patent Document 2: K. T. Park, K. Hiraga and M. Sagawa, “Effect of Metal-Coating and Consecutive Heat Treatment on Coercivity of Thin Nd—Fe—B Sintered Magnets,” Proceedings of the Sixteen International Workshop on Rare-Earth Magnets and Their Applications, Sendai, p. 257 (2000), and Non-Patent Document 3: K. Machida, H. Kawasaki, S. Suzuki, M. Ito and T. Horikawa, “Grain Boundary Tailoring of Nd—Fe—B Sintered Magnets and Their Magnetic Properties,” Proceedings of the 2004 Spring Meeting of the Powder & Powder Metallurgy Society, p. 202. Since Dy or Tb is more effectively concentrated at grain boundaries, this method succeeds in increasing the coercive force without substantial sacrifice of remanence. This method is applicable to only magnets of small size or thin gage for the reason that as the magnet has a larger specific surface area, that is, as the magnet is smaller in size, a larger amount of Dy or Tb is available. However, the application of metal coating by sputtering poses the problem of low productivity.
Patent Document 3: WO 2006/043348A1 discloses means for efficiently improving coercive force which has solved the foregoing problems and lends itself to mass-scale production. When a sintered R1—Fe—B magnet body, typically sintered Nd—Fe—B magnet body is heated in the presence of a powder on its surface, the powder comprising one or more of R2 oxides, R3 fluorides, and R4 oxyfluorides (wherein each of R1 to R4 is one or more elements selected from among rare earth elements inclusive of Y and Sc), R2, R3 or R4 contained in the powder is absorbed in the magnet body, whereby coercive force is increased while significantly suppressing a decline of remanence. Particularly when R3 fluoride or R4 oxyfluoride is used, R3 or R4 is efficiently absorbed in the magnet body along with fluorine, resulting in a sintered magnet having a high remanence and a high coercive force. In Patent Document 3, since absorption treatment is carried out on the magnet surface, the magnet body to be treated is prepared by machining a sintered magnet block to a predetermined shape. The dimensions of the magnet body are not particularly limited. The patent describes: “The amount of R2, R3 or R4 absorbed into the magnet body from the powder deposited on the magnet surface and comprising at least one of R2 oxide, R3 fluoride and R4 oxyfluoride increases as the specific surface area of the magnet body is larger, i.e., the size thereof is smaller. For this reason, the magnet body includes a maximum side having a dimension of up to 100 mm, preferably up to 50 mm, and more preferably up to 20 mm, and a side having a dimension of up to 10 mm, preferably up to 5 mm, and more preferably up to 2 mm in the direction of magnetic anisotropy. Most preferably, the dimension in the magnetic anisotropy direction is up to 1 mm.” This intends absorption treatment over a wider region of the magnet body. In Example, a magnet body which has been machined to the final shape is subjected to absorption treatment. In the permanent magnet rotary machine, however, the area susceptible to demagnetization is only a portion of magnet, suggesting that the high coercive force portion need not necessarily account for the major region of a magnet body. Finishing to the final shape means that small magnet bodies are to be handled, giving rise to the problem that the process efficiency is not increased due to difficulty of handling.
An example is taken to illustrate that the area susceptible to demagnetization in the permanent magnet rotary machine is localized rather than the entirety of magnet. In AC servo motors, for example, a permanent magnet rotary machine with a radial air gap as illustrated in FIG. 4 is used. This permanent magnet rotary machine comprises a rotor 3 including a rotor core 1 and permanent magnet segments 2 attached to the surface of the core, and a stator 13 surrounding the rotor 3 to define a gap therebetween and including a stator core 11 having a plurality of slots and coils 12 wound on teeth. In the permanent magnet rotary machine illustrated in FIG. 4, the number of permanent magnet poles is six (6), the number of teeth is nine (9), and the arrow associated with a permanent magnet segment indicates a direction of magnetization thereof. With regard to the permanent magnet segments, magnetic orientation is effected in a parallel magnetic field so that a direction of easy magnetization is parallel to the center axis of the magnet segment. The coils are wound on teeth as a concentrated winding and connected in Y connection of three phases: U, V and W phases. The solid circle of a coil denotes that the coil winding direction is forward and the crossing (X) of a coil denotes that the coil winding direction is backward, with respect to the plane of paper.
In AC servo motors and similar motors requiring high precision torque control, the torque must have less ripples. Accordingly, it is undesired that when the permanent magnets rotate, the alignment of stator slots and the permanent magnets causes cogging torque to develop due to variations of the magnetic flux distribution across the gap (i.e., torque without current flowing across the coil) or torque ripples to occur when driven by current flowing across the coil. The torque ripples exacerbate controllability and additionally, cause noise. The cogging torque may be reduced by configuring a permanent magnet segment to a cross-sectional shape that tapers from the center toward transverse ends as shown in FIG. 4. With this configuration, the end portion of a permanent magnet segment which is a magnetic pole switch area developing a noticeable variation of magnetic flux distribution produces a smoothened magnetic flux distribution, reducing the cogging torque.
When electric current flows across coils, magnetic fields are developed in the directions of broad arrows depicted in the stator core region, so that the rotor is rotated counterclockwise. At this point, an aft area of a permanent magnet segment in the rotating direction (a circled area in FIG. 4) is in a situation susceptible to demagnetization because the magnetic field is in an opposite direction to the magnetization of the permanent magnet segment. Demagnetization would not only reduce the driving torque, but also give rise to the problem of increased cogging torque due to locally uneven magnetic field.